OVERLAPPING FORMS 153 



forms. Yet, though much remains to be done in clearing up the 

 facts, one thing is certain, namely that the neutral zone has not a 

 definite and normally intermediate population, but on the con- 

 trary it is peopled by fragments of the two definite types and 

 miscellaneous mongrels between them. 



On the other hand, one cannot readily suppose that either 

 form was the parent of the other. The process must have 

 involved both addition and loss of factors, for whatever hypoth- 

 esis be adopted, such changes must be supposed to have occurred. 

 A careful statistical tabulation of the way in which the characters 

 are distributed in the population of the mixed zone would be 

 of great value, and till that has been done there is little that can 

 be said with certainty as to the genetics of these characters. 

 In the collection of Dr. Bishop of New Haven I was very kindly 

 allowed to examine a sample, all taken at random, near together, 

 in Saskatchewan. There were females 4 adult, 2 young; males 

 4 adult and 5 young. This number, though of course insufficient, 

 is enough to give some guide as to the degree of definiteness which 

 the characters generally show in their variations. Of the 15 

 birds, 8 had simply yellow quills; 2 had red; 1 was almost red 

 but had one yellow tail-quill; 3 were intermediate and 1 was 

 buff. As regards the malar patch, which can only be determined 

 properly in the adult males, 1 was red, 1 was approximately red, 

 2 intermediate. As to nuchal crescent 4 females had none, 2 

 females very slight; 7 males had it, 1 had only a slight crescent, 

 and 1 had none. In point of quills therefore 10 were definite 

 out of 15; in point of crescent, 11 were definite out of 15; and in 

 point of malar patch 1 only was definite out of 4. The last is a 

 feature directly dependent on age and so counts for less, but as 

 regards the other two features there is some indication that the 

 factors show definiteness in their behaviour. It must be re- 

 membered that we have no knowledge what the heterozygous 

 form may be, and in the case of red and yellow it is probably a 

 reddish buff. The patch-works are no doubt to be compared with 

 other well-known pied forms, and in these we must suppose the 

 active factor broken up, which it probably can be very easily. 

 The asymmetry, which Allen notices as so marked a feature, in the 



